1. Field of the Invention
The invention relates to a secondary air supply apparatus for supplying secondary air to an upstream side of an emission control apparatus disposed in an exhaust system of an internal combustion engine and method for detecting an abnormality of the secondary air supply apparatus, more particularly, to a secondary air supply apparatus and method capable of detecting an abnormality of a component part of the apparatus.
2. Description of the Related Art
In a known emission control apparatus of an internal combustion engine, a three-way catalyst is disposed in an exhaust system in order to lessen the CO, HC and NOx components in exhaust gas. In a known technology for enhancing emission control, air is delivered by an air pump into a secondary air supply passage equipped with an open-close valve connected to an exhaust pipe, so that the secondary air supplied into the exhaust pipe increases the oxygen concentration and therefore causes oxidation of HC and CO in exhaust gas.
If a component element of the secondary air supply apparatus, such as the air pump, the open-close valve, etc., has an abnormality, the exhaust gas cleaning efficiency will reduce, and emission will deteriorate. Therefore, it is necessary to determine such an abnormality at an early time. Known technologies for detecting an abnormality as mentioned above are, for example, a technology disclosed in Japanese Patent Application Laid-Open No. 9-21312, and a technology disclosed in Japanese Patent Application Laid-Open No. 9-125945.
In the former technology, a pressure sensor is disposed between the open-close valve and the air pump in the secondary air supply passage, and the pressure value detected by the sensor is used as a basis for detecting an abnormality in the secondary air supply apparatus. In the latter technology, a pressure sensor is disposed in the secondary air supply passage, and an abnormality in the secondary air supply apparatus is detected on the basis of a difference between the detected maximum and minimum values of pressure pulsation.
However, according to these technologies, although an abnormality of the secondary air supply apparatus can be detected, it is difficult to precisely determine which one of the component elements is abnormal. Furthermore, in the case of a malfunction where a component element does not normally function but the pressure value or the pressure pulsation value is normal, the abnormality cannot be detected.